The invention relates to a process for making a thermoplastic molding composition and more particularly to extruding a material system that contains significant amount of water.
A process for producing a thermoplastic molding composition is disclosed. The process entails introducing a solid polymeric resin, optionally containing residual moisture, into an extruder, operating under conditions designed to melt the resin and charging water into the melt while in the extruder to yield wet, molten resin that contains water in an amount of at least about 2 percent. The wet, molten resin is then devolatilized to yield dry resin having water content not exceeding 0.3 percent. Molding compositions thus produced have improved properties. In a preferred embodiment, the process results in a molding composition that contains a fine dispersion of a discontinuous phase in a continuous polymeric phase.
The art has long recognized that some physical properties, notably yellowness index, of a material system, such as a thermoplastic molding composition, deteriorate in the course of extrusion. It is also well recognized that material systems that contain phases that are immiscible one in the other could form fine dispersions upon intensive mixing in the course of extrusion. Some of the mechanical energy imparted to the system in the course of extrusion converts to excess thermal energy that could result in the deterioration of the resin and a corresponding increase in its yellowness index. In practice, those skilled in the art have learned to achieve a compromise between good color and fine dispersion. The inventive process makes it unnecessary to sacrifice good color (low yellowness index) in favor of good dispersion.
Of possible relevance to the present invention is U.S. Pat. No. 5,753,169, where a method for reducing the amount of unreacted unsaturated carboxylic acid or its derivatives from a grafted polyolefin is disclosed. This method entails melt kneading, for instance in an extruder, in the presence of water, a modified polyolefin. The polyolefin is grafted with an ethylenically unsaturated carboxylic acid or of its derivatives. Also noted is U.S. Pat. No. 5,354,804, where a method of emulsifying a thermoplastic resin composition has been disclosed. The process, carried out in an extruder, entails adding water down stream from the melt zone. EP 56123 is noted disclosing the preparation of a molding composition by mixing in the melt a linear saturated polyester and a rubbery graft copolymer. The process includes using a graft copolymer that contains water in an amount of 5 to 50% by weight and requires the polyester to be introduces in melt form.
The present invention resides in the finding that good color and fine dispersion can be attained simultaneously upon the addition of water to a material system in the course of its extrusion.
In accordance with the invention, a material system, preferably a thermoplastic molding composition, is produced in a process that includes the following steps:
(i) introducing a solid polymeric resin that may optionally contain residual amount of water into an extruder operating at conditions designed to produce a melt of said polymeric resin;
(ii) introducing an additional quantity of water, preferably water at room temperature, into said melt in an amount calculated to bring the water content of the melt to at least about 2 percent, preferably about 2 to 10, more preferably about 2 to 8 percent, to produce a wet melt; and
(iii) devolatilizing said wet melt to produce a dry melt having a water content no greater than 0.3 percent.
The percents, all occurrences, being relative to the weight of the polymeric resin, the polymeric resin of the invention includes no polyolefin that is grafted with ethylenically unsaturated carboxylic acid or its derivatives. The solid polymeric resin introduced in step (i) of the inventive process may contain water in an amount that is preferably lower than 0.8, more preferably lower than 0.5, percent.
The material system produced by the present invention preferably contains at least one polymeric resin as a continuous phase and at least one discontinuous, or dispersed phase that is immiscible in the continuous phase.
Included within the scope of the continuous polymeric resin are virtually any thermoplastic resins. Among these, mention may be made of polyolefins, polystyrenes and copolymers of styrene and/or xcex1-methyl styrene including copolymers with acrylonitrile (SAN). Most preferably, the discontinuous phase includes any of (co)polybutadiene, (co)polyacrylate, styrene-butadiene rubber (SBR), ethylene-propylene (EP) and ethylene-propylene-diene monomer (EPDM) rubbers. Most preferably, the material system includes acrylonitrile-butadiene-styrene (ABS), acrylonitrile-acrylate-styrene (ASA) and acrylonitrile-EPDM-styrene (AES).
The immiscible, discontinuous phase of the material system, may also be an additive or filler, typically an inorganic filler or pigment or gel.
The extruder useful in the practice of the invention is a well-known machine that is widely used in the preparation of thermoplastic molding compositions. It is characterized in that it continuously mixes and kneads the components of a material system material. The preferred extruder is equipped with two or more unidirectionally rotating screws in tight mesh with each other at least in the input zone and the dispersion zone of the device. Extruders have excellent dispersion capabilities and also effect a satisfactory mixing within a narrow spectrum of dwell time within the device. The extruder suitable in the context of the present invention is equipped with means of introducing water. The introduction of water may be carried out at a single point or, in the alternative, at a plurality of points along the extruder Further, the introduction of water is preferably post-flux, that is after the completion of the melting process. The devolatilization is carried out under vacuum vent(s). The operating pressure is at, or less than, atmospheric pressure, preferably at, or less than, 4.0 psia.
The advantages to be attained by the process of the present invention are expressed in terms of low yellowness index and good dispersion of the resulting material system. Yellowness index is determined in accordance with ASTM D 1925. The degree of dispersion in the present context is determined by visual count of the un-dispersed particles that are greater in size (largest dimension) than 0.012 inches that are contained within a 48 square inches area (24 inches in length and 2 inches wide) of a thin (20-25 mils) sheet produced by extrusion in a conventional, single flight, single screw, plasticating extruder.